Étendue is a property of an optical system. It is given by the cross-sectional area of a cone of light (in the plane perpendicular to the propagation direction), times the solid angle subtended by the light. Other names for this property are acceptance, throughput, light-grasp, collecting power, and the AΩ product. Étendue is important because it never increases in any optical system. A perfect optical system produces an image with the same étendue as the source.
The importance of étendue is in determining optical system throughput. Any given source emits light into an optical system with efficiency dependent on the system étendue. Most surface emitting light sources (e.g., light emitting diodes) do not match the étendue required of projection panels. The die are either too small, requiring the addition of die, or too large, requiring some sort of aperture to reduce the LED étendue to match the panel étendue.
Given the aforementioned shortcomings, it would be desirable to match LED die to panel étendue in order to maximize optical system throughput.